This invention relates to a centrifugal blower of low static pressure.
A centrifugal blower comprises an impeller mounted in a casing for rotation to draw a fluid, for example, air, through a suction port of the casing axially into the blower and blow the fluid by the vanes centrifugally into the casing, so that the fluid is blown in a predetermined direction through a discharge port open at one side of the casing. The majority of the fluid, blown into the casing by traversing the vanes, is guided by the casing and blown in the predetermined direction through the discharge port. However, part of the fluid blown into the casing in this way flows through the casing and a clearance between a suction member of the casing and a shroud of the impeller to the suction side of the casing. Because the fluid once drawn into the casing leaks to the suction side, the performance of the blower is reduced and the power required to operate the impeller increases. The air leaking to the suction side produces noises.
In view of the aforesaid problems encountered in centrifugal blowers of the prior art, it has been earnestly desired to reduce the volume of air leaking to the suction side of the blower after being blown into the casing. The performance of a blower can be improved by reducing the size of the clearance between the suction member and the shroud. However, since the suction member is stationary and the shroud rotates, the minimum size of the clearance is determined by the precision with which the suction member and the shroud are fabricated and assembled. Thus, there is a limit to the improvement in the performance of the blower that can be achieved by reducing the size of the clearance.
Proposals have been made to provide means for preventing air from leaking into the clearance between the suction member and the shroud in addition to reducing its size. U.S. Pat. No. 3,782,851 discloses a centrifugal fan device wherein walls project in staggered relation into a clearance between the suction port and the shroud to thereby increase the length of the path through the clearance, so as to thereby increase the resistance offered by the path to the flow of air and hence to minimize the volume of air leak passing through the clearance.
U.S. Pat. No. 3,842,902 discloses an axial blower in which each blade has an annular seal member attached to its forward end, and the seal members and the shroud are formed in various shapes to avoid the flow or air leak by the forward ends of the blades.